Some quick help please

since you found the gears cheap enough, That's a good deal.
Had you not; I would have said to replace just second gear, and the bent mainshaft(if in fact it is bent).
And the reason is this;
First gear is very lightly loaded.
Fourth is even more lightly loaded. Usually this gear ios not even used until cruising speed.
Third is a between gear. It may or may not see hard work depending on how willing you are to get caught speeding.
Second is the workhorse gear. It almost always gets you to 60/65 mph at peak horsepower or a bit beyond. And when you get into it at the top of first gear, you will be right on peak-torque, so again it is taking abuse.
And if you punch it at 35mph in second, you will be doing about
3000/3100 with 3.55s. By 45 or so you will be at peak torque. So again second is taking a licking.
But I've seen worse; still in operation.
Honestly,IMO,that tranny is way over-engineered for SBM-A-bodies. Just look at the top-loaders from the 60s and 70s. Or the pussy Saginaws. I bet two of those just barely out weigh an A833